Hospitals, schools, hotels, offices, factories and other like buildings are commonly provided with so-called fire doors which, although they may have only a limited resistance to combustion, also serve the purpose of restricting the spread of smoke and/or the supply of combustion air in the event of a fire. Because of this latter purpose, it is desirable that all fire doors in a building should be closed immediately a fire breaks out even if such doors are remote from the seat of the fire. For this reason, it is preferred that fire doors be kept shut at all times. However, in practice, they are frequently wedged open to permit free passage.